A NOVEL MUSCLE-SPINDLE CONTAINING MUSCLE-FIBERS DEVOID OF SENSORY INNERVATION IN THE POSTERIOR CRICOARYTENOID MUSCLE OF THE NORMAL ADULT GUINEA-PIG

A novel muscle spindle composed of five thin intrafusal muscle fibres and four thick muscle fibres was encountered in the posterior cricoary tenoid muscle of the normal adult guinea pig. Four intrafusal muscle f ibres of the five were innervated by the annulospiral type of sensory ending, but lacked a distinct aggregation of equatorial nuclei such as bags and chains. On the other hand, one intrafusal fibre and four thi ck fibres between the layers of the spindle capsule absolutely lacked sensory innervation and were almost similar in the fine structural fea tures to neighbouring extrafusal fibres. These fibres are considered t o fail to receive the sensory innervation in an early stage of develop ment, and to retain an undifferentiated feature of intrafusal fibre in the intracapsular space into adulthood or to follow the course of ext rafusal differentiation between the layers of the spindle capsule. It is suggested therefore that this muscle spindle may be primitive.